The blast furnace process is the prevailing method of primary iron production. Considering the large investment in blast furnaces, it is doubtful that the technology will be replaced in the next 20 years, the life expectancy of the present blast furnaces.
Coke is an essential blast furnace feed stock. Coke serves as a fuel, to supply heat and to produce a reducing atmosphere, and contributes to the permeability of the furnace burden. Coke is produced by the destructive distillation of coal. In the distillation process, coal is heated in an oven in the absence of air (oxygen). The volatile matter (methane, hydrogen, tar, benzene, ammonia, naphthalene, etc.) is driven off leaving behind a solid material comprising mostly carbon, which is called coke.
The volatiles produced from early coke making facilities were burned as they evolved from the coke ovens. The recovery of the volatiles for use as chemical feed stocks and fuel was a significant early advancement in coke making technology. The recovered coal chemicals, which include: coal tar, light oil (benzene, toluene, xylene), naphthalene, and ammonia, were the foundation of the chemical industry for the first half of this century. Coke oven gas, consisting of methane, hydrogen and the other gaseous hydrocarbons that remained after the coal chemicals were removed, was distributed through pipelines to neighboring industries and towns to be used as an industrial and domestic fuel. Many coke plants were operated primarily for the recovered chemicals and coke oven gas, the coke being a secondary product that contributed little to the profitability of the plant.
In recent years, the chemical industry has shifted away from coal chemicals and is now based primarily on chemicals produced from petroleum and natural gas. Natural gas has become the industrial and domestic fuel of choice, eliminating the need for coke oven gas. Coke ovens now exist to supply coke for the blast furnace and to a much lesser extent the foundry industry. The recovered coal chemicals are commonly sold at a loss considering the high cost of operating and maintaining the chemical recovery plant (referred to as the by-product plant by the iron and steel industry where the primary product is coke).
The energy required to operate a by-products plant is about one half the value of the recovered chemicals and coke oven gas. Continued large capital investment is also required to up-grade by-product plants so that they comply with increasingly stringent environmental requirements.
Considering that there is little to be gained by recovering coal chemicals, developments in coke making technology have reverted back to non-recovery principles such as described in U.S. Pat. No. 4,287,024. Once again, coke ovens are being designed to burn the volatiles as they evolve from the ovens. However, to achieve an energy efficiency approaching that of a recovery coke making facility, the products of combustion of the volatiles are conducted under the ovens so that the heat required for the coking process can be extracted. The combustion products are then ducted to waste heat recovery boilers where much of the remaining heat is extracted to produce steam. The steam is used for power generation and heating. The modern non-recovery coke oven technologies are based on special oven design. It is not possible to convert or retrofit existing recovery coke ovens to these non-recovery technologies.
Other attempts in dealing with coke oven gases for purposes of combustion and/or elimination of toxic components are described in U.S. Pat. Nos. 3,292,345; 3,581,715; 4,100,033 and 5,433,600. U.S. Pat. No. 3,292,345 describes the combustion of ammonia and H.sub.2 S gases removed from the coke oven gases by way of a stripping operation. This approach does not replace the by-products facility but instead only replaces one unit of that facility in dealing with the ammonia gases. U.S. Pat. No. 3,581,715 describes the production of calcium carbides in the use of a continuous travelling grate stoker. The temperatures and products of combustion in making the calcium carbide are cooled before providing supplementary fuel for a steam generator. U.S. Pat. No. 4,100,033 describes the immediate combustion of volatiles as they are emitted during the coke oven charging step. This set-up is not designed for continuous operation in treating coke oven gas during the entire coking process. U.S. Pat. No. 5,433,600 describes a burner for burning coke oven gases. The burner construction is designed for burning coke oven gases derived from a by-products facility. The small diameter fuel orifices and swirling elements in the fuel stream would tend to clog due to all of the tar and fines constituent in the raw coke oven gases and not provide satisfactory service.
The life expectancy of existing recovery coke ovens is between 5 and 20 years. Because of the large capital cost of coke plants, these plants will continue to operate to the end of their lives to supply blast furnace coke. By design, the recovery coke oven requires the operation of the associated by-product plant. The by-product plant provides the following necessary operating requirements:
1) Suction required to remove the volatiles from the ovens. PA1 2) Pressure control for the coke ovens. The pressure on the ovens must be precisely controlled so that it is slightly negative to prevent leakage of volatiles and to limit the amount of air infiltration. PA1 3) The production of coke oven gas to underfire the ovens. PA1 4) Eliminate the coal chemicals in an environmentally acceptable manner. PA1 5) Achieve an acceptable plant energy efficiency for economical and environmental reasons. PA1 i) a battery of coke ovens; PA1 ii) a plurality of ascension pipes connecting the coke ovens to a raw coke oven gas collector main, the ascension pipes and collector main having means for spraying flushing liquor to cool the raw coke oven gases emitted from the coke ovens and collect sprayed flushing liquor in said collector main; PA1 iii) a transport main for transporting flushing liquor and raw coke oven gas from the collector main to a burner of a steam producing boiler, the transport main including a downcomer for separating raw coke oven gas from flushing liquor; PA1 iv) the burner supplying air for mixing with raw coke oven gas delivered by the transport main to effect burning of raw coke oven gas in the boiler and produce a boiler exhaust; PA1 v) a suction fan being provided at an exhaust of the boiler to draw boiler exhaust from the boiler and develop thereby a negative pressure in the coke ovens, ascension pipes, collector main, transport main, burner and boiler, the boiler having reinforced walls of substantially gas-tight construction to withstand negative pressure, raw coke oven gas in said transport main is drawn through said burner by such negative pressure developed in said boiler by said suction fan; PA1 vi) means for detecting a change in negative pressure in the system; and PA1 vii) a controller system for varying draw of the suction fan in response to a detected change in negative pressure in the system. PA1 i) a negative pressure reinforced boiler with suction fan for exhausting combustion gases from the boiler with, PA1 ii) a burner introducing cooled separated raw coke oven gas under negative pressure for burning in the boiler, PA1 iii) means for controlling air to fuel ratio to said burner, and PA1 iv) means for varying negative pressure developed in said boiler by said suction fan. PA1 i) burning a cooled separated raw coke oven gas stream at a negative pressure by drawing combustion gases from the boiler with a suction fan.
The non-recovery coke making system of this invention supports the operation of existing or new recovery type coke oven batteries by providing the above listed essential operating requirements as a by-product plant. However, they are provided in a completely different manner.
The volatiles from recovery coke ovens, which equate to the raw coke oven gases, have never been considered to be a practical fuel. No technology exists to handle the volatiles at a temperature high enough to prevent the heavy components of the tar and pitch from condensing. Present technology for cooling and transporting the volatiles to the by-products plant is not compatible with present fuel systems equipment. The severe fouling conditions caused by condensing of tar, pitch, and naphthalene, and coke, coal, and ash particulate, quickly fouls piping and in line devices. There is no pumping or compressing equipment technology, no flow metering technology that can operate under these condition with the reliability required for combustion safety in a boiler system.
The non-recovery system of this invention enables the volatiles from the coke ovens to be burned in power boilers to produce steam at a high efficiency. The process involves burning the volatiles in a combustion process designed to incinerate the volatiles and to minimize formation of pollutants that could form due to the chemical composition of the volatiles. The boilers efficiently extract the heat of combustion from the combustion products. The clean, cool products of combustion can be passed through suction fans to provide the suction required for the coke oven battery operation.
The proposed non-recovery cokemaking technology can be retrofitted to existing recovery coke ovens. Increased thermal efficiency and the relative greater value of power compared to coal chemicals makes such retrofits economically attractive. For new installations, the capital cost is much less than for the construction of a by-products plant.